This invention concerns an improved method for visualizing a coronary arterial tree known as a roadmap. Such a roadmap is useful during coronary angioplasty operations. In coronary angioplasty a steerable guidewire is guided into specific arterial branches of the heart. A dilation balloon is then directed along the guidewire, placed in position relative to the stenotic or abnormally narrow segment of the artery and then expanded to dilate the artery. Coronary angioplasty is used as an alternative to coronary artery bypass surgery for improving blood flow in patients with coronary artersclerosis.
Steerable guidewires have greatly expanded the use of angioplasty, but correct positioning of the guidewire is hampered by inadequate means to simultaneously visualize the coronary arteries. The methods developed to date largely focus on displaying a static display of the coronary arterial tree taken after injection of a visible contrast medium, over a live fluoroscopic image of the beating heart. Since the contrasting image is static and the live image is dynamic, this overlay method results in substantial misregistration discrepancies. In other words, the static roadmap representation is out of phase with the beating heart during most of the cardiac cycle. This renders accurate visualization difficult.
U.S. Pat. No. 4,204,225, assigned to the Wisconsin Alumni Research Foundation, discloses a method of producing television difference images from an X-ray image of an anatomical subject. The object of this invention is to show circulating blood with enhanced visibility while substantially eliminating bone and soft tissue from the images through subtraction.
The method disclosed includes integrating a mask or initial X-ray image over a predetermined number of television fields before an injected X-ray contrast medium becomes visible. This integrated mask image is then subtracted in real time from later incoming television fields taken after the contrast medium is visible, ultimately producing difference video images. These difference video images represent changes in the X-ray images subsequent to taking of the mask.
Because this method utilizes an integrated mask, it does not take into consideration the motion of the beating heart and accordingly, considerable misregistration errors still exist.
U.S. Pat. No. 4,204,226 assigned to the Wisconsin Alumni Research Foundation is very similar to U.S. Pat. No. 4,204,225. This patent includes creation of a series of television difference images which represent changes in the X-ray image between successive time intervals.
Neither of the above two patents include the step of ECG synchronization to correlate the live and mask images. Generally, cardiologists and radiologists are used to seeing live X-ray images, and viewing live real time subtracted images is distracting and not generally helpful clinically. As a result, the techniques of the above two patents have largely been clinically abandoned.
European patent No. 0,193,712 to Siemmens discloses an X-ray diagnostic device for subtraction angiography. This invention also utilizes a single mask frame and integrates it over a full cycle. By integrating the mask over the full cardiac cycle, the misregistration error is simply distributed and not minimized.
A need therefore exists for a method to produce a representation of the coronary arterial tree which when added to the live fluoroscopic images follows the movement of the heart with a minimum of misregistration. Use of this method would better assist angiographers in accurate placement of guidewires and dilation balloons during coronary angioplasty operations.